The backscatter method of determining the density or moisture content of a material comprises exposing the surface of a material to direct radiation from radioactive source and measuring the backscatter radiation with a detector at an external position which is shielded from direct radiation. The source typically comprises a radioisotope, such as Radium, which emits gamma photons and the detector is selected to be responsive to photons and is coupled to a counter for totaling the photons impinging on the detector during a predetermined time period. The gamma rays entering the material are scattered by the electrons of the material and the number of photons which are directed toward the detector provide an indication of the density of the material when compared to a calibration curve for the particular instrument. A direct relationship exists between a material's density and the amount of radiation it will absorb and scatter.
In one prior art method of determining density by backscatter measurement, the instrument is placed in direct contact with the material. This method is not entirely satisfactory because the density measurement is subject to two variables: surface irregularities and chemical composition. Surface irregularity represents a serious obstacle to accurate measurement when density readings are taken of materials such as concrete, asphalt, soil, aggregetes and the like. Surface preparation is required to smooth out the irregularities as much as possible, and the surface preparation is time consuming and requires static readings at the prepared surface. The chemical composition of the material affects the respective amounts of radiation that will be absorbed and scattered. For example, high silicon content materials and high iron content materials may have approximately the same density but will provide different meter readings with the contact method. This prior art method included the use of a calibration curve for each type of material. However, the chemical composition of a material may not always be known.
To eliminate chemical variation errors inherent in the contact process, an air gap ratio method was developed. This method includes the taking of a first reading with the meter in direct contact with the material and a second reading with an air gap between the meter and the material. When the contact reading is made, the number of photons counted by the detector is a function of the chemical composition, density and surface irregularity of the material. When the air gap reading is taken, however, the number of photons counted by the detector is functionally related more closely to the composition of the material. As a result, the density of the material can be determined irrespective of chemical composition by comparing the ratio of the air gap reading to the contact reading. The chemical factor is eliminated so that the ratio can be compared to a calibration curve for the instrument to provide a density reading. The air gap-contact process however, is still sensitive to surface irregularities, so that accurate measurement still required time consuming surface preparation.
A further advance in this art was made when it was discovered that, if both backscatter measurements are taken at preselected distances from the surface of the material, the surface irregularity variable could also be eliminated to provide more accurate final readings. Instead of the contact measurement just described a measurement is taken at a location just above the surface. A ratio is then established and compared to a calibration curve to provide a final density reading.
While the latter method has overcome some of the initial problems in the development of this art, the equipment used for taking the measurements has not developed at the same rate. Because radioactive materials are employed in the measuring process, great care must be taken to properly shield the source material not only from the detector but from the operator of the instrument. In addition, if the air gap method is to be employed, the instrument must be precisely located at the predetermined distances from the surface of the material. The instrument should also indicate at all times whether the source is in a use or a "safe" position and the source should automatically be returned to the safe or storage position when the instrument is being moved, shipped or stored. An instrument which satisfies these criteria would be a significant advance in this technology.